Progressively collapsible variable resistance element

ABSTRACT

A resistance element of elastic material has at least an interconnected portion coated with a nonfriable electrically conductive material forming an electrical path that changes its resistance as a function of the state of tension or compression of the resistance element. A conductive material is prepared from particulate electrical conductive material, an elastic binder and a solvent. Pretreatment of an elastic material of elastomer foam prior to application of conductive material produces a useful resistance element.

Un ted States Patent [1113,629,774

[72] inventor Nelson A. Crltes {56] References and N in???" UNITEDSTATES PATENTS [211 P 2,042,606 6/1936 Kotowski 252/511 [22] Filed Oct.21,1968

2,472,214 6/1949 l-lurvitz 338/36 X [45] Patented Dec. 21,1971 [73]Assignee Scientific Advances, Inc. 2,679,569 5/1954 Hall 117/226Continuation-impart of application Scr. No. 217341978 2/1956 338/2 X523,205, Jan. 26, 1966, now abandoned, 310991573 7/1963 17/226 'l Pp fiPrimary Examiner-Lewis H. Myers 6092372 1967 MW Fbmdoned- AssistantExaminer-D. A. Tone This application Oct. 21, 1968, Ser. No. Ammy (;ray,Mase' and Dunson 772,464

- ABSTRACT: A resistance element of elastic material has at [54]PROGRESSIVELY COLLAPSIBLEVARIABLE least an interconnected portion coatedwith a nonfriable elec- RE I TA ELEM trically conductive materialforming an electrical path that Chin" 26 Drum! changes its resistance asa function of the state of tension or 52] us. c1 338/114, compression ofthe resistance eme t. A conductive material 117/226, 252/502, 338/99 isprepared from particulate electrical conductive material, an 511 1111.011101c 13/00 elastic binder and a solvent e atment of an elastic materi-501 r1610 61 sun-611 333/2, 36, al of elaswmer foam p i t pplication ofconductive materia1 produces a useful resistance element.

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NELSON A. CRITES ATTOR N EYS RESISTANCE.

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LDADING UNLOADING DEF'LECTION mils INVENTOR. NELSON A. CRITES ATTORNEYSx- FOAM DENSITY a5 ||O/fi (AR/F RANGE, 0-200) FOAM DENSITY lO.9|b/ (AR/FRAN E, -loo) 50%LOADING PATENTEI] 0622] I9" w m w m w w w. w \mE;o $2

PER CENT BY WElGHT INVEN TOR. NELSON A. CRITES BY MiG/,4. y ,21 3

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PATENTED M221 1971' SHEET 5 BF 7 I B D w\ INVENTOR.

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BY GRAY MASE & DUNSON PATENTEllnficansn. I 3629.774

" SHEET8UF 7 f Fig. 24

INVENTOR NELSON A. CRITES BY GRAY, MASE a DUNSON ATTORNEYS PATENTEDBEBZIan 3.629.774 'SHEET 7 OF 7' ,Fig. 25

INVENTOR NELSQN A. CRITES BY GRAY,MASE 8 DUNSON ATTORNEYS PROGRESSIVELYCOLLAPSIBLE VARIABLE RESISTANCE ELEMENT CROSS-REFERENCE TO RELATEDAlPLlCATlON This application is a continuation-in-part of copendingapplication Ser. No. 609,372 filed Jan. 16, 1967, now abandoned whichwas a continuation-in-part of application Ser. No. 523,205 filed Jan.26, 1966, now abandoned.

BACKGROUND OF THEINVENTION This invention relates to a resistanceelement and method of making a conductive material and, moreparticularly, this invention relates to an elastically collapsibleresistance element having an electrical resistance varying with thestate of tension or compression of the resistance element. Thisinvention further relates to low-cost transducers constructed from theaforesaid resistance element.

There is a need for low-cost devices which when included in anelectrical circuit change their electrical properties when disturbedmechanically or produce an e.m.f. as a result of a disturbance by ormovement relative to a surrounding medium. These devices find especiallywidespread use for transducers. Known devices are available that areselected on the basis of the specific quantity to be measured and theenvironment surrounding the device. Unfortunately, most of the knowndevices capable of meeting the requirements of a given environment areexpensive. Forexample, strain gages electrically responsive to a changeof length are used in a variety of transducer configurations. Inaddition, certain rare earth compounds are known to be electricallyresponsive upon application of pressure and have found use in transducerconstructions. Although accurate, the cost of each of the above devicesis prohibitive where it is desired to use a low'cost device to measureprimarily quantitative differences in many units such as the measurementof oil pressure in the engine block of an automobile. A low-cost devicethat has been used in the latter applications includes an arm which isactuated mechanically to traverse a coil connected in an electricalcircuit. The movement of the arm and the resistance of the circuitchange with the amount of force applied to the arm. These devices arecomplicated by the number of parts required and often fail as a resultof material breakdown.

Two recent approaches to the problem of providing a lowcost transducerare based on the use of resistance elements of nonmetallic materialsthat are rendered electrically conductive. Deutschle (U.S. Pat. No. 3,0ll,063) shows a transducer comprising a sheet member of mechanicallydeformable material having an electrical resistance that varies with thestate of defonnationof the material. Conductive rubber is found to be asatisfactory material. Although this approach has merit, it has beenfound that an elastomer and an electrically conductive material cannotbe combined so as to leave the elastic and electrical properties of eachmaterial unaffected by the presence of the other. Bulgin (U.S. Pat. No.2,734,978) describes coating the surfaces of the cell walls of anonconducting elastic cellular material with an adherent coating of apulverulent electrically conductive material such as powdered carbon.Some difficulty arises in connection with use of the resistance elementsof Bulgin as a result of flaking or spalling of the pulverulent materialfrom the surfaces of the cells upon deformation thereof. Further, theelements have only one specific range of resistance and a definitesensitivity and thus are limited to applications wherein thissensitivity and range of resistance is satisfactory. The low-costtransducers made from the aforementioned resistance elements suffer fromthe additional disadvantages of low output and lack of temperaturecompensation.

Accordingly, it is an object'of this invention to provide a simplelow-cost resistance element. It is a further-object of this invention toprovide a method of making a conductive material.

It is a still further object of this invention to provide a resistanceelement including elastic and electrically conductive material in amanner so as to leave the properties of the former unaffected by thepresence of the latter.

It is another object of this invention to provide a resistance elementincluding elastic and electrically conductive portions in an integralstructure that will not loose its mechanical or electrical propertiesafter repeated deformations.

SUMMARY The present invention provides a resistance element whichtypically comprises an elastic material having a progressivelycollapsible skeletal structure and a nonfriable elastic electricallyconductive continuous coating adherent to at least an interconnectedportion thereof, the coating having an average thickness of about 0.05to 0.25 of the thickness of the skeletal structure, and the elementfilling less than about 25 percent of the volume within its outerboundary. The coating comprises an elastic binder and electricallyconductive particles dispersed therein to form a substantially smoothouter surface at which the conductive particles are substantially, butnot completely, embedded. The conductive material typically comprisesabout 10 to 25 percent of the volume of the coating.

A preferred form of progressively collapsible variable resistanceelement according to the invention comprises a threedimensional networkof interconnecting strands of elastic material integrally interconnectedby nexuses at spaced points to form the skeletal outline of a multitudeof polyhedrons each face of which is polygonal and common to a pair ofadjacent polyhedrons, substantially all of the faces being open and freefrom membranous elastic material; and a nonfriable elastic coating,substantially covering the interconnecting strands and nexuses withoutsubstantially filling the open polygonal faces, which comprises anelastic binder and electrically conductive particles dispersed thereinto form a substantially smooth outer surface at which the conductiveparticles are substantially, but not completely, embedded in the binder.The average thickness of the coating is generally less than about 0.5 ofthe radius of the interconnecting strands and the network typically hasabout 10 to I00 polygonal faces per lineal inch. There may also be atleast one intermediate layer of elastic material, preferably siliconerubber, between the skeletal network and the coating. The skeletalnetwork is preferably formed of a polyurethane resin and the conductiveparticles are preferably carbon.

Another useful form of the progressively collapsible variable resistanceelement may comprise an interwoven structure of fibrous strands, a layerof elastic material substantially covering those strands and bondingthem at spaced points of intersection to form a three-dimensionalinterconnected network substantially free of membranous material in theareas between strands, the elastic layer imparting elasticity to thestructure as a whole, and a nonfriable elastic coating substantiallycovering the network without substantially filling the areas betweenstrands, which comprises an elastic binder and electrically conductiveparticles dispersed therein to form a substantially smooth outer surfaceat which the conductive particles are substantially, but not completely,embedded in the binder.

The invention also includes a method of making a nonfriable elasticcoating having conductive particles dispersed therein to form, whenapplied, a substantially smooth outer surface at which the conductiveparticles are substantially, but not completely, embedded whichcomprises the steps of heating the conductive particles under vacuum toremove impurities, cooling the conductive particles to a predeterminedtemperature insufficient to cause vaporization of a selected liquid,adding to the conductive particles, to form a paste, a liquid that doesnot vaporize at that temperature, adding a flowable elastic material tothe paste to form a conductive composition, mixing the conductivecomposition to disperse the conductive particles uniformly therein, andadding a catalyst to the mixed composition to promote cross-linking ofthe elastic material.

Additional liquid may be added to the conductive composition aftermixing to reduce its viscosity to a level that enables the compositionto penetrate freely into substantially all of the open cellular regionsof an elastic porous mass. The liquid may be a solvent or an emulsifyingagent.

Also included is a method of making a progressively collapsible variableresistance element from a substantially completely open-celled,three-dimensional network of interconnecting strands of elastic materialwhich comprises applying to at least a portion of the outer surface ofthe network a flowable nonfriable conductive coating comprising anelastic binder and electrically conductive particles dispersed thereinto form a substantially smooth outer surface at which the conductiveparticles are substantially, but not completely, embedded, spreading thecoating on the network until it is evenly distributed over the outersurface thereof, centrifuging the network to remove excesscoa ting, anddrying and curing the coated network to set the elastic binder. Thenetwork may be saturated with a liquid before applying the coating.

In the drawings:

FIG. 1 is a sectional view of a relaxed resistance element according tothe invention;

FIG. 2 is a sectional view of the resistance element of FIG. 1 in acompressed state;

FIG. 3 is a sectional view of another resistance element according tothe invention;

FIG. 4 is a sectional view of still another resistance element accordingto the invention;

FIG. 5 is a sectional view of yet another resistance element accordingto the invention;

FIG. 6 is a sectional view of still another resistance element accordingto the invention;

FIG. 7 is a sectional view of another resistance element according tothe invention;

FIG. 8 is a sectional view of still another resistance element accordingto the invention;

FIG. 9 is a perspective view,of yet another resistance element accordingto the invention;

FIG. 10 is a perspective view partially in cross section of a transduceraccording to the invention;

FIG. 11 is a perspective view partially in cross section of anotherresistance element for the transducer of FIG. 10;

FIG. 12 is a sectional view of a load cell made from the transducer ofFIG. 10.

FIG. 13 is a perspective view of a Bourdon tube made with read-out meansincluding a transducer according to the inventron;

FIG. 14 is a sectional view of a temperature compensated transduceraccording to the invention;

FIG. 15 is a sectional view along the line 15l5 of FIG. 14;

FIG. 16 is a cross-sectional view of a high output temperaturecompensated transducer according to the invention;

FIG. 17 is a cross sectional view of another high output temperaturecompensated transducer according to the invention;

FIG. 18 is a graph of the hysteresis behavior of a transducer made fromthe resistance element according to the invention;

FIG. 19 is a graph of the sensitivity or change of resistance per unitof force applied to a variety of resistance elements made according tothe invention;

FIG. 20 is a graph of the sensitivity or change of resistance per changeof unit length for a variety of resistance elements made according tothe invention.

FIG. 21 is a perspective view of a transducer having controlledresistance characteristics according to the invention.

FIG. 22 is a cross-sectional view of another transducer havingcontrolled resistance characteristics according to the invention. I

FIG. 23 is a photomicrograph (20X) of a section of a coated, reticulatedpolyurethane foam resistance element according to this invention.

FIG. 24 is a photomicrograph (IOOX) of a cross section of one of theinterconnecting strands of the resistance element shown in FIG. 23.

FIG. 25 is a photomicrograph (200X) of a portion of the cross section ofa strand such as shown in FIG. 24, showing the conductive coating.

FIG. 26 is aphotomicrograph (5,000X) taken to the surface of theconductive coating on an interconnecting strand such as shown in FIG.25.

The invention includes within its scope a resistance element and methodof making a conductive material wherein the resistance element includesan elastic material having a progressively collapsible structure havingat least an interconnected portion thereof coated with a nonfriableelectrically conductive material. In one embodiment, the conductivematerial forms an electrical path through the aforesaid resistance ele-1 ment. It has been found that the use of the aforesaid resistanceelements in a low-cost transducer obviates many of the difficultiesattendant upon low-cost transducers heretofore in use and provides manyadvantages.

According to the invention, the elastic material can be made from avariety of materials and may assume many dif ferent configurations. Theelectrically conductive coating material must be nonfriable but mostimportantly should be firmly adherent to the elastic material. Animportant feature of this invention is that the composition of theconductive coating material can be tailored to the environment in whichit is to be used. The conductivity of the conductive coating and thedegree of its influence on the elasticity of the elastic material areselected with reference to the sensitivity of the system of which it isto comprise a component member. For example, where high sensitivity isrequired, the conductivity is relatively low and the coating exertslittle or no influence on the elastic behavior of the elastic material.V

The requirement that the conductive coating material be adherent to atleast a portion of the interconnected portion of a structure having theability to collapse or extend progressively upon compression or tensioncan be better understood by reference to FIGS. 1 and 2. In FIG. I, across section of an embodiment of a resistance element selected forpurposes of illustration is in a relaxed position. The element comprisesa cellular foam having the cell walls 13 of the interconnected cells 12bonded with a conductive coating 15. The structure of FIG. 1 iselastically compressed in FIG. 2 causing cell walls 13 of the relaxedstructure to collapse so that the space in each cell 12 decreases aswell as the ratio of void to solid space in the cellular element. Itwill be obvious that this decrease of void space will occurprogressively with compression of the cellular element. When conductivecoating 15 is adherent to cell walls 13 according to the invention, thecloser proximity of areas of conductive coating 15 upon compressiondecreases the length of the current path and thus the electricalresistivi- I ty. The conductive coating 15 yields with the movement ofcell walls 13 as a result of being nonfriable and adherent. It is notnecessary that coating 15 be as elastic as the material of the cellwalls 13 because a firm bond'of coating 15 to elastic cell walls 13insures elastic movement of the entire structure. Upon relaxationpf thefoam of FIG. 2, interconnected cells 12 will open progressively and theelectrical resistance will increase until it reaches the relaxedposition of FIG. I. Extension or stretching of the relaxed structure ofFIG. 1 will cause a further increase of resistance. Thus, the resistanceelement is suitable for measurement of either tensile or compressiveforces.

THE ELASTIC MATERIAL While a cellular structure such as is present in anelastic foam has been shown in FIGS. 1 and 2 for purposes ofexplanation, numerous other materials and configurations of materialsare suitable for the practice of this invention. The only requirement isthat the interconnected portion of the elastic material adherent to theconductive coating collapses or extends progressively in relation todeformation or extension of the elastic material. Satisfactory elasticmaterials include those having the inherent characteristics of highresilience (low elastic modulus) or those having a high elastic modulusbut formed into a highly resilient shape.

Referring to FIG. 3, an elastic material comprises a porous body ofsmall irregularly shaped particles 26 held together at points or smallcontact surfaces 28. In a typical embodiment, each particle 26 may havean interconnected cellular network as exemplified in the photomicrographof FIG. 23. A conductive element is prepared in one method by firstbonding the contact points by preliminary coating of the particles witha bonding material and subsequent compaction to the desired level ofporosity. The degree of porosity depends on the size required for thevoids 22. The bonded porous mass is then impregnated with a conductivecoating material 25 which bonds to the free surfaces defining voidspaces 22. Alternatively, the bonding material used for preliminarycoating of the particles can also serve as the conductive coating wheresatisfactory bonding can be achieved at the contact points with theconductive coating. Upon compression of the porous mass, conductivecoating 25 is brought into tighter contact over larger areas byprogressive closing of void spaces 22 to cause a change of electricalresistance.

In FIG. 4, another porous body comprises a plurality of hollow spheres36 bonded at their contact points 38 and defining the void spaces 32,the spheres having thereon a conductive coating 35. Instead of hollowresilient spheres, the resistance element of FIG. 4 can comprise solidspheres of resilient material unitized in a porous mass such as the bodyof irregular particles of FIG. 3. Further, the body can comprise aplurality of spheres or irregular shapes of sponge rubber bonded attheir contact points. In addition to spheres, cylindrical shapes orellipsoids, prolate spheroids, etc., randomly disposed in a porous massand bonded at their contact points provide a suitable material.

In FIG. 5, intermingled fibers, flakes or shells'46 form a strongelastic fibrous unit containing therein void spaces 42. To form athree-dimensional network capable of the elastic movement required forthe elastic material according to the invention, the fibers or flakes 46are interlocked along their length either by chemical combination, bymeans of secondary forces, or by mechanical entanglements. A mass ofrandomly oriented steel fibers can be interlocked by mechanicalentanglement. By first coating the fibers with a solution of latex orother bonding material, such as silicone rubber, chemical combination isused to interlock the fibers. In addition to steel wire, numerous otherfiber materials are satisfactory. For example, hogs hair coated with alatex solution can provide an elastic material having excellentresilience. A typical bonded junction of fibrous strands as in FIG. 5may appear in cross section as in the photomicrograph of FIG. 24.

For a material having a high elastic modulus, a variety ofconfigurations can provide the high resilience needed for the practiceof the invention. Referring to FIG. 6, a metal comprises a plurality ofoverlapping portions of alternate flat layers 66 and corrugated layers67 wherein the walls of the corrugated layers form cells 62 having across-sectional area increasing with distance from the point of contact68 of corrugated layers 67 and fiat layers 66. The angle defined at theintersection of fiat layers 66 with corrugated layers 67 should berelatively low to insure a progressive closing or opening of cells 62upon compression of extension.

Referring to FIG. 7, a modification of FIG. 6 is a metal foil having aplurality of overlapping portions 74 forming stacked cells 72 having adepth increasing with distance from the point of contact 71 with itsadjacent cell. A portion of the metal inside of cells 72 is embossed sothat ease of contact is assured upon progressive closing or opening ofcells 72 upon compression or extension of the elastic body.

In one embodiment, the elastic body can merely comprise a series ofstacked embossed plates bonded at the point of contact with theiradjacent plates. Referring to FIG. 8, superimposed plates 76 havingembossed portions 77 are arranged so that the embossed portion 77contacts a flat portion on a lower adjacent plate at contact point 78.The space between the embossed portion and the flat portion of an upperadjacent plate forms cells 79. Because the stacked plates are notcontinuous in FIG. 8, only the underside of the plates is coated withconductive material. The current path changes when the points of contactprogressively enlarge upon compression of the body. Another resistanceelement similar to that of FIG. 8 can be made by using alternatecorrugated and fiat plates rather than the continuous metal foil asdescribed in FIG. 6. On the other hand, the structure of FIG. 8 can bemade continuous by interconnecting the plates as shown in FIGS. 6 and 7.In the latter case, a first insulating coating which may be formedmerely by oxidation of the metal is covered with a firmly adherentconductive coating.

In the elastic materials of FIGS. 1 through 8, an interconnectedcollapsible structure has a plurality of cellular units that collapse orextend progressively in relation to the applied force. The use of aplurality of cells, each opening or closing progressively withcompression or extension, provides excellent reliability in comparisonto a configuration where, say, a single larger collapsible cell is used.The effect provided in the multicellular structures can be provided in astructure of the latter type wherein each portion having the ability toopen or close progressively in relation to applied force runs the fulllength of the resistance element.

Referring to FIG. 9, a plurality of springs 52 of varying pitch areplaced in spaced relation to one another. Each coil 56 of a spring 52 isspaced from its neighboring coil by a distance that decreases from oneend of the spring to the other end. For example, the space definedbetween coil 56A and 56B is greater than the space defined between coil56B and 56C. By providing a varying pitch, progressive engagement ofadjacent coils is insured upon compression of spring 52. The closestcoils engage one another upon initial compression and the progressiveengagement continues as a function of the coil spacing after the firstcoils become fully engaged. 0n release of the compression, progressiveopening of the spring occurs. In spring 52 of FIG. 9, after the coilsbelow 56C are in full engagement, the circumference of coil 56Cprogressively engages the circumference of coil 568. When coils 56C and56B are in full contact, coil 56B begins to progressively engage thecircumference of coil 56A. When a conductive coating 55 is bonded to thecoils of springs 52 according to the invention, the electrical path isprogressively shortened upon compression of springs 52 and theelectrical resistance decreases. In actual use, springs 52 areelectrically interconnected by means not shown. The springs can be madefrom an electrical conductor such as steel or from an electricalnonconductor such as fiberglass.

Where the configurations of FIGS. 5, 6, 7, 8, and 9 are metals, anadherent conductive coating having a high resistance relative to that ofthe metal is used. In this way, changes of dimension of the elasticmaterial that affect the resistance of the resistance element arereadily apparent. Where the conductive coating can form a current paththrough the structure, a first insulating coating is applied to themetal. The conductive coating is then applied over the first insulatingcoating. Where the configurations of FIGS. 1 through 5 arenonconductors, electrically conductive material can be incorporated inthe nonconductor. In most cases, the amount of electrically conductivematerial will be small so as to not adversely affect the elasticproperties of the nonconductor. In either event, an adherent conductivecoating must be applied to the nonconductor.

When using a body of elastomer foam for the elastic material of FIGS. 1and 2, commercially available elastomer foams of silicone rubber knownas Silastic R'IV Silicone Rubber (trade name of Dow Corning Corporation)and RTV 7 Silicone Rubber (trade name of General Electric Company) aresatisfactory. The foremost requirement for the nonconducting foams isthat the cells should be interconnected so that an electrical path isavailable upon application of the conductive coating. A foam ofcontrolled cell size is easily prepared by mixing appropriate quantitiesof elastomer and sugar or salt crystals or other soluble crystals ofpredetermined uniform size. When the rubber sets, the crystallinematerial is leached away to leave a foam body. Similarly, a heatdecomposable material can also be used to locate the ultimate cell sitesin the foam body. Manual working of the foam ruptures sufficient cellwalls to provide the interconnection needed for an electrical path.

A useful body of elastomer foam comprises a skeleton foam of a firstresilient material uniformly coated and impregnated with a secondresilient material. The composite foam thus formed is used where a foammaterial is needed for its properties such as resilience or resistanceto chemical attack but cannot be obtained commercially in a uniform cellsize. For example, the properties of silicone rubber render it desirablefor a foam material. Often, it cannot be obtained as a foam with auniform cell size. Accordingly, it has been found that a suitablecomposite foam is available from a commercial polyurethane foam ofuniformcell size having the pores thereof coated with three to fourtimes its weight of silicone rubber. The composite has better resiliencyand resistance to chemical attack than a polyurethane foam alone and amore uniform cell size than a silicone foam alone. To make the foregoingcomposite foam, polyurethane foam is impregnated with a solution ofsilicone-acetic anhydride-xylene mixture. The xylene is released alongwith the acetic anhydride and the silicone rubber vulcanizes in placearound the polyurethane skeleton.

THE CONDUCTlVE COATING As previously discussed, the conductive coatingof this invention must be: (1) capable of being adherent to the elasticmaterial or forming a firm bond therewith; (2) nonfriable; and (3)electrically conductive. In the preparation of the conductive element,the elastic material is ordinarily dipped in a solution of theconductive coating. Where demanded by the shape of the elastic material,impregnation is done by capillary action and can be aided by applicationof a vacuum. A satisfactory conductive coating includes an elastomercombined with a particulate electrical conductor. A variety ofelastomers and electrical conductors can be used. For example,elastomers would include RTV (room temperature vulcanizable) siliconerubber, natural rubber latex, polyurethane rubber, etc. A satisfactoryRTV silicone rubber is Clear Seal (trade name of General ElectricCompany). Electrically conductive materials include carbon, graphitizedor partially graphitized carbon, silver, gold, copper, tungsten,aluminum, and various metals and alloys used alone or in combinationwith one another. An example of a satisfactory electrically conductivematerial of carbon is Conductex SC (trade name of Columbian CarbonCorporation). Conductex SC is a carbon black which has a mean particlediameterof 170 A. Extremely fine particles such as these are desirablein that they allow a more uniform conducting surface to be obtained.

Another important aspect of the conducting coating of this invention isthat the outer surface of the coating is substantially smooth. There areno brittle conducting fiber ends protruding for some distance from thesurface which can break off during compression or expansion of theresistance structure. Also, the conducting particles are not merelybonded onto the surface of the coating where they can abrade off duringdeformation. in the present invention the conducting particles which aregenerally very fine, are substantially, but not completely, embedded inthe coating surfaces such that they will not break or abrade off. Thisallows resistance elements according to this invention to be used forprolonged periods of time without significant changes in resistancevalues or in linearity.

The amount of electrically conductive material used in the conductivecoating or its loading and the amount of coating applied to the elasticbody will be dictated by the electrical characteristics and mechanicalproperties desired in the final product. By electrical characteristicsand mechanical properties, it is meant to refer to the sensitivity ofthe resistance elementor the change of resistance per unit length orforce, and the hysteresis behavior of the resistance element, as well asthe actual resistance of the resistance element in its relaxed position.For example, where the resistance element is being used as a transducerto measure relatively high pressure, it will be desired to use arelatively high modulus element. Where pressures are low, a low moduluselement having a broad range of resistance is desired to pick up slightchanges in the surroundings.

The resistance of the conductive coating is readily controlled by theamount used as well as by its loading. The modulus of the elasticmaterial can be influenced by the amount of coating used. The effect ofthe conductive coating on the modulus of the elastic material depends tosome extent on the nature of the elastic material. The effect is morepronounced in the case of a low modulus material (e.g., elastomer foam)than it is in the case of a high modulus material (e.g., metal spring).In tailoring the properties of the resistance element to therequirements of a particular application, there will be some overlapupon adjusting the variables of loading and amount of coating. For agiven loading of electrically conductive material in the coating,increased amounts of coating material increase modulus and also decreaseresistance because the effective amount of electrically conductivematerial present in the resistance element increases. Where highermodulus is desired at a high resistance the loading of the coatingshould be minimized. Where low modulus is desired with low resistance,the loading of the conductive coating must be increased. Theinterrelationship of the variables that are adjusted in tailoring theconductive coating will become more apparent upon examination of theexamples to follow.

MAKING A CONDUCTlVE COATING One preferred method of making a conductivecoating comprises forming a dispersion of particulate electricallyconductive material in a liquid containing an elastic material.Conductive coating made in this manner are well suited for resistanceelements according to the invention.

The preferred method of producing an elastic, nonfriable conductivecoating includes the steps of:

a. heating the conductive particles under vacuum to remove impurities;

b. cooling the conductive particles to a predetermined temperatureinsufficient to cause vaporization of a selected liquid;

c. adding to the conductive, particles, to form a paste, a liquid thatdoes not vaporize at said temperature;

d. adding a flowable elastic material to the paste to form a conductivecomposition;

e. mixing the conductive composition to disperse the conductiveparticles uniformly therein; and

f. adding a catalyst to the mixed composition to promote cross-linkingof the elastic material.

Generally, the final coating composition has a thick, pasty consistencyand is therefore difficult to work into the inner cellular regions of anelastic porous mass. To facilitate penetration into these inner cellularregions it is desirable to add more liquid to the composition,preferably the same liquid as is used to form the conductive particlepaste. This liquid is preferably added after the mixing step wherein theparticles are uniformly distributed and serves to reduce the viscosityof the final coating, thereby enabling the coating to penetrate freelyinto substantially all of the open cellular regions.

Generally, the conductive particles constitute about 10 to 25 percent ofthe volume of the coating. When carbon particles are used, they mayconstitute about 20 to 50 percent of the coating by weight. It has beenfound that where the coating contains a very small amount of conductiveparticles, it will not produce the required resistance characteristics,and that where it contains too many particles, the elasticity of thecoating is decreased to an extent which will undesirably affect theelement as a whole. Therefore, the amount of conductive particles in thecoating (also referred to as loading) must be within certain prescribedranges to be useful.

The liquid carrier for the elastic material can be either a solvent oran emulsifying agent. In the preferred method a solvent such as naphtha,xylene or other aromatic hydrocarbon, is employed. The total amount ofsolvent used is dictated by the amount of coating desired for theparticular application. Increased amounts of solvent cause greaterdilution or lesser amounts of coating after the solvent is driven off.The solvent is driven off after the coating is applied to the elasticbody by the application of moderate amounts of heat.

When using an emulsifying agent, a procedure is followed similar to thatdescribed for the solvent. In this case, however, finely divided elasticmaterial is mixed with an emulsifying agent. The liquid containing anelastic material is then intermixed with liquid comprising a dispersionof electrically conductive material.

Instead of mixing each of the components of the conductive coating alonewith a solvent prior to subsequent intermixing, it is often desirable tointermix all the components with a solvent in one batch.'This procedureis enhanced when used together with ball milling or roller milling. Thelatter procedure insures a fine particle size of carbon and aids theuniform dispersion thereof in making a resistance element. Theconductive coating should be applied to the elastic material as soon aspossible after mixing and before the cross-linking of the elasticmaterial has gone to completion. Resistance elements preparedaceordingto the foregoing procedures appear to have improved propertiesin relation to those prepared from coatings prepared using separatemixing steps.

' MAKING A RESISTANCE ELEMENT Often, particularly where theconfigurations of FIGS. through 9 are metals, the elastic structure iscoated merely by dipping in the conductive coating. But where theelastic structure consists of a substantially completely open-celled,threedimensional network of interconnecting strands, such as areticulated elastomer foam, it is often .difficult to obtain completepenetration of the coating and at the same time prevent clogging of thepores. To overcome these problems, it is preferred to apply the coatingaccording to the method defined by the following steps:

a. applying to at least a portion of the outer surface of the network aflowable nonfriable conductive coating comprising an elastic binder andelectrically conductive particles dispersed therein to form asubstantially smooth outer surface at which said conductive particlesare substantially, but not completely, embedded;

b. spreading the coating on the network until it is evenly distributedover the outer surface thereof;

c. centrifuging the network to remove excess coating; and

d. drying and curing the coated network to set the elastic binder. i

The centrifuging step is important in that it removes any excess coatingmaterial that may be clogging the pores of the structure and alsoremoves excess coating material of the interconnecting strandsthemselves such that only a thin layer of coating is left on eachstrand. It has been found that the coating must have an averagethickness of about 0.05 to 0.25 of the thickness of the skeletalstructure and that element comprising the skeletal structure with itsadherent coating must fill less than about 25 percent of the volumewithin its (the element) outer boundary. By adjusting the duration ofthe centrifuging step, the thickness of the coating may be controlledwithin the desired limits.

When using an elastic material of elastomer foam having a large numberof interconnected cells, impregnation can be difficult especially wherea conductive coating of high conductivity is desired. In the latterinstance, the outer cells tend to become clogged with the high viscositycoating containing a large percent of conductive material. Attempts touse greater amounts of solvent to carry the conductive coating tends toresult in the formation of microcracks upon evaporation of the solvent.

It has been discovered that when the elastic material comprises anelastomer such as a foam of silicone rubber, the pretreatment thereofwith a solvent such as naphtha, xylene or other aromatic hydrocarbonallows the use of coatings of high conductivity.

According to the pretreatment, a portion of the solvent used to carrythe conductive coating is eliminated for use as a carrier andsubstituted in the pretreatment operation. The elastomer is allowed tosoak in the solvent until it is saturated. In making a resistanceelement from an elastic material of elastomer foam, the pretreatment isobserved to 'cause swelling of the foam. The larger pores thus formedallow easier penetration of the foam upon impregnation with conductivematerial immediately following the solvent pretreatment.

The solvent pretreatment is found to have advantages with regard to theelectrical characteristics of the resistance element as well as itsmechanical properties. The former advantage finds application in theproduction of resistance elements of elastomer foam whereas the latterapplication has even wider application.

With regard to electrical characteristics, the elimination of a portionof the solvent from the conductive coating results in a mixture of pastyconsistency. Because of the decreased solvent requirement, greateramounts of coating can be used to provide a higher effective carboncontent than previously available without pretreatment. Previously, theneed for high solvent limited the amount of coating that could be usedthereby requiring increased loading of conductive material in thecoating to provide higher conductivity. It will be apparent that withthe solvent pretreatment, higher conductivity coatings can be made withreduced amounts of conductive material.

The benefits of pretreatment on mechanical properties relates to theinteraction of the conductive coating with the elastomer wall. Theadvantage derived renders the pretreatment suitable for preparingresistance elements from a composite of solid elastomer and conductivecoating as well as from an elastomer foam. The solvent used forpretreatment apparently does not evaporate until after vulcanization ofthe elastomer of the conductive coating thereby minimizing thepossibility of microcracking. The reduced amount of solvent needed tocarry the conductive coating also reduces the possibility ofmicrocracking from this source. Another surprising advantage relating tomechanical characteristics is found to result upon evaporation of theabsorbed solvent. The elastomer body shrinks to compress the coating andin effect prestresses the coating. As stated previously, the latteradvantage makes the pretreatment process especially useful forresistance elements comprising a solid elastic substrate coated with aconductive coating wherein expansion or contraction of the compositecauses the resistance of the conductive coating to changeproportionally.

THE TRANSDUCER Where the elastic material is a metal such as in FIGS. 6,7, or 8, a transducer is provided by affixing electrical leads to thetop and bottom metal portions and including the resistance element in asuitable electrical circuit. In the embodiment selected for illustrationin FIG. 10, an elastic foam body has an interconnected cellular portion82 coated with a conductive coating 85. While an elastic material ofnonconductive foam is shown in FIG. 10, it is not meant to be limitedthereto. Any of the elastic materials of FIGS. 3 to 5 or FIG. 9 may beused. Contacts 89 are affixed to the longitudinal ends of foam 80 bymeans of an electrically conductive cement 88. A satisfactory cementcomprises equal proportions by volume of a solvent release cement havingthe ability to shrink upon curing and a mixture of silver flake andsilver powder. Wolfson et al. (US. Pat. No. 2,774,747) described atypical conductive cement. Leads affixed to contacts 89 connecttransducer 84 in an electrical circuit comprising a power source andelectrical readout device. As explained in connection with FIGS. 1 and2, when foam 80 changes dimension by reason of physical force appliedthereto, there is a corresponding change in the current path orelectrical resistance measured by the electrical readout device.

Although an elastic material can be used alone as in FIG. 10, the use ofa solid elastic material such as liver rubber in combination therewithrelieves a portion of the stress. In addition, foam 80 of FIG. can beimpregnated to a limited depth only so that only a portion of the foamis axially traversed by conductive material.

Referring to FIG. 11, the axial core 96 of foam 90 having a conductivecoating on the interconnected cell walls thereof receives a solidelastic material 97. Contacts (not shown) are affixed to ends of thecomposite body as described in connection with FIG. 10. The solidelastic material 97 of FIG. 11 may comprise live rubber or any otherhighly resilient material such as a fiberglass spring or metal springinsulated from the conductive coating. In addition to the core of FIG.11, solid elastic material may be a coaxial portion such as a sleevesurrounding the foam body. The solid elastic material may be combinedwith any of the coated elastic materials such as those illustrated inFIGS. 1 through 9.

Referring to FIG. 12, a load cell 110 comprises a transducer 104 insideof a bellows 108 having the ability to deform elastically uniformly in alongitudinal direction with applied pressure. The contacts 106 oftransducer 104 are insulated from bellows 108 by insulating discs 109.Leads 101 from a power source (not shown) carry current to thetransducer and are insulated from bellows 108. By enclosing transducer104 within load cell 110 to measure fluid pressure, the transducer isleft unaffected by the presence of the fluid. Further, the deformationof the transducer can be controlled by the elastic properties of thebellows. Thus, FIG. 12 provides a method of controlling the modulus ofthe elastic material in addition to the previously described method ofadjusting the amount of conductive coating.

A further application for the transducer of this invention includes theBourdon tube 120 of FIG. 13 wherein a transducer 124 is grasped betweenthe opposing faces 116 and 118 of Bourdon tube 120. Insulating plates125 serve to insulate the contact caps 127 from faces 116 and 118.Transducer 124 is connected to a power source (not shown) by leads I21.Fluid enters faces I16 and 114 and flows to closed face 118 wherebypressure in tube 120 causes face 118 to move outwardly to release forceon'transducer 124 in an amount proportional to the fluid pressure. Inturn, force released from the transducer causes the length of thecurrent path flowing in the transducer circuit to change. When fluidpressure decreases, the faces move inwardly and force is exerted on thetransducer. Where desired, the mechanical arrangement normally used inthe Bourdon tube to transmit the changes of tube geometry to a pointeror readout device can be included with the arrangement of FIG. 13. Inthis manner, two types of readout are provided.

For many applications, the transducer according to this invention showsremarkably little change in properties when subjected to varyingtemperatures. However, where close accuracy is needed, temperaturecompensation is usually desired. Referring to FIGS. 14 and 15, The outerresistance elements 135 and 136 surround the shorter inner resistanceelements 131 and 132. The upper end of inner resistance elements 131 and132 and outer resistance elements 135 and 136 are affixed to aninsulating plate 138. Insulating plate 139 is affixed to the lower endsof outer resistance elements 135 and 136. Leads 141 from a power source(not shown) are attached to resistance elements 135 and 136. Outerresistance elements 135 and 136 are responsive to pressure whereas innerresistance elements 131 and 132 are not. Leads 142 through 146interconnect the resistance elements so that each element comprises onearm of a Wheatstone Bridge circuit. Each of the inner elements 131 and132 is connected to one of the outer elements and 136 so that an innerelement and an outer element comprise alternate arms in the bridgecircuit. Because the voltage drop across them is equal to the appliedvoltage, their outputs are added and the output is doubled. Althoughwires can be used to interconnect the resistance elements in the bridgecircuit, some leads can be printed on the inner face of the contacts byconventional techniques. To insure maximum output on the bridge circuit,the resistance of all the resistance elements should be equal. In otherwords, the fixed resistance of the elements not responsive to pressureshould be equal to the normal resistance of the elements responsive topressure. By normal resistance, it is meant the resistance of theresponsive elements prior to being placed in the environment in whichthe transducer operates. This is done according to the invention byvarying the composition and amount of conductive coating applied to theresistance element. The inner elements do not have to compriseresistance elements according to the invention so long as theirresistance is equal to the normal resistance of the outer elements andthey respond to temperature in the same manner as the outer elements. Aload cell can be made by including the temperature-compensatedtransducer of FIGS. 14 and 15 within the bellows of FIG. 12.

Referring to FIG. 16, a temperature-compensated transducer has fourtimes the output of a transducer comprising one element alone. Thetransducer comprises the upper resistance elements 161 and 162 and thelower resistance elements 168 and '169 affixed to the end caps 159 and171, respectively, at one end and affixed at their opposing ends to aninsulating plate 165 having a pressure applying disc 166 affixed to itsrim. The transducer assembly is disposed inside of a bellows assembly179 and separated therefrom at the longitudinal ends by the insulatingdiscs 157 and 173. At a point about midway along the bellows, pressureapplying disc 166 is affixed to the bellows assembly 179 to define anupper bellows portion 155 and lower bellows portion 177. The tabs 153are affixed to the upper and lower faces of bellows 179. The resistanceelements are connected by electrical leads 175 so that each resistanceelement comprises one arm of a Wheatstone Bridge arranged with upper andlower resistance elements as alternate anns. In actual use, spaced rodsare run through the openings in tabs 153 so that the position of thetransducer is fixed and pressure of the surrounding environment istransmitted solely to pressure applying disc 166. Leads from a powersource (not shown) are connected to end caps 159 and 171. Uponapplication of pressure to pressure applying disc 166 in a directionshown by the arrows, the lower bellows portion 177 and lower resistanceelements 168 and 169 are compressed while the upper bellows portion 155and upper elements 161 and 162 are placed in tension. Preferably, thenormal resistance of the resistance elements of FIG. 16 is about equal.

Referring to FIG. 17, a modification of the transducer of FIG. 16comprises the transducer assembly enclosed within a rigid container 198and insulated therefrom by the insulating plates 191 and 199. A rod 192extends through an opening in the top of the rigid container 198 and isaffixed at its base to insulator plate 193. Pressure responsive means189 are provided on the portion of rod 192 outside of rigid container198. Upon application of force to the pressure applying means 189, thetransducer assembly 195 responds in the manner described for FIG. 16.The resistance elements of FIGs. 16 and 17 that are stressed in tensionmay initially be in the relaxed state or compressed (prestressed). Thelatter alternative is useful where the resistance material has a lowtensile strength. The most useful resistance element of FIGS. 1 through9 for the transducers of FIGS. 12 through 17 will be determined by theproperties sought to be measured and the accuracy desired.

13.-. EXAMPLE 1 The hysteresis behavior of atransducer according to theinvention was studied by measurements of change of resistance with forceupon application of a load to a given load of force and upon release ofthe load to a relaxed state. Resistance elements for the transducer weremade using a silicone rubber foam having a density of 8.5 lb./ft.and alength of one-half inch and a diameter of one-half inch. Conductivecoating was prepared by mixing VM&P (Varnish Makers and Painters)naphtha (Standard Oil Company of Ohio) with silicone rubber (GE ClearSeal) and particular carbon (Conductex SC Beads) respectively inseparate containers. The total amount of naphtha used was governed bythe amount of coating desired. The respective solutions were mixed,stirred thoroughly, and resistance elements were prepared by dippingfoam bodies in the solutions described below:

Following application of the conductive coating, the resistance elementswere placed in a warm oven until the solvent was driven off.Electrically conductive cement was used to affix brass contacts having adiameter of about five-eighths inch and a thickness of about 0.050inchto each end of the resistance element. Leads were affixed to theelectrodes so as to complete an electrical circuit including a powersource and electrical readout device. Load was applied to eachtransducer and the electrical resistance measured at variousdeflections. Upon full compression of the resistance element, load wasgradually released in small increments so that deflection and resistancecould be recorded. The graph of FIG. 18 shows the hysteresis behavior ofthe three transducers. Excellent reliability is shown for the numerousranges of resistance that are covered.

EXAMPLE 2 The useful resistance ranges of resistance elements made inthe manner of the transducers of example I silicone foams of twodifferent densities were determined in relation to the conductivity ofthe coating and amount of conductive coating used. Measurements ofelectrical resistance were made in the relaxed state and in the fullycompressed state. The tabulation below compares the resistance rangeswith the amount and conductivity of the conductive coating.

2| l I00 so: 10410 a 1 3 I00 on 12-400 2|: I00 45.5 52-230 s l 5 I00 1:zoo-2,000 215 I00 33.: 120-670 301 so us 100400 201 so 11.: 320-355 305so 12.: zso-isoo 205 so 41.: iso-ooo 301 so 55.1 l.3006.700 207 so 214$604,500

ZOO-Density of Foam equals l0.9 lit/ft. 300-Demity of Fonrn equals 8.5lbJft.

From the above, it is apparent that a wide variety of ranges ofelectrical resistance are available by adjusting the amount ofconductive material in the coating and the amount of coating used.

EXAMPLE 3 Transducers were made in the manner described for example landthe relationship of force to change of resistance per unit of forceapplied and change of resistance per change of unit length were studiedfor various amounts and conductivities of conductive coatings. In thisway, the effect of the latter variables on sensitivity could be studied.Referring to FlGs. l9 and 20, the sensitivity of the resistance elementsvaries within a wide range depending on the amount and type ofconductive coating that is used. This allows conductive elements to betailored for specific environments.

EXAMPLE 4 The useful resistance ranges of resistance elements werestudied as a function of the manner of applying a conductive coating.Resistance elements were made from an elastic material of siliconerubber foam having a density of 8.5 lb./ft. and a length ofthree-eighths inch and a diameter of 1% inch. Conductive coating forsample 101 was prepared by mixing xylene with silicone rubber (GE ClearSeal) and particular carbon (Conductex SC Beads) and ball milling forSminutes. Immediately following ball milling, the resistance element wasprepared by dipping the foam body in the coating solution. For sample103, percent of xylene that was used for the conductive coating ofsample 101 was used to impregnate the foam body prior to dipping it in acoating solution containing 80 percent less xylene than was used for theconductive coating of sample 101. Transducers were prepared in themanner described for example land measurements of electrical resistancemade in the relaxed state and with an applied force of 50 grams.

The formulations and electrical properties of the transducers are givenbelow:

The use of solvent pretreatment for sample 103 provided a resistanceelement having improved conductivity in relation to sample 101. Sample103 was cycled several times with no change in the resistance from thatshown above. The ball milling procedure is also shown to provide higherconductivity as evidenced by comparing sample 303 of example 2 withsample 101.

Often it is desirable to provide a transducer having a variable modulusso that the electrical resistance characteristics of the transducer upondeformation can be closely controlled. This allows adjustment of thesensitivity of the transducer within various deformation ranges.Referring to FIG. 21 a pair of stacked verticalresistance elements 212and 214 to form the resistance element 216 are provided between theelectrical contacts 219 affixed to each of the longitudinal ends of theresistance element 216. The elements 212 and 214 are affixed to oneanother and to the contacts 219 by means of electrically cosductivecement 218 Where resistance elements 212.

and 214 are of different modulus, the lower modulus material collapsesrapidly upon initial compression of the transducers to cause animmediate change of resistance. The element of higher modulus collapsesmore slowly and causes a further change of resistance with deformationafter the lower modulus element has ceased to be effective.

Referring of FIG. 22, a transducer 225 having controlled resistancechange with deformation comprises a resistance element 227 of elastomerfoam between electrical contacts 237 forming a central opening 229provided with an insulated spring 231 and a resilient stop 233. Initialdeformation of the transducer 225 causes a change of resistance dictatedby the characteristics of the resistance element 227. Upon furtherdeformation, the upper contact 237 strikes spring 231 whereby itcontrols additional deformation of the resistance element 227. Whendeformation equals the distance between the end of stop 233 and thelower electrical contact 237, the stop controls further deformation. Ateach of the latter two stages, the transducer can be subjected to higherdeformation thereby extending its useful pressure operating range.

A preferred embodiment of the variable resistance element according tothe present invention is shown in FIGS. 23 to 26. This embodimentcomprises a commercial polyurethane foam having a first coating ofsilicone rubber which is in turn coated with an elastic electricallyconductive material. FIGS. 23 to 26 are photomicrographs of increasingmagnification which show in greater detail the structure of the element.

FIG. 23 shows the cellular structure of a reticulated polyurethane foamcoated with a conductive coating according to this invention. It isclear from this photomicrograph that the coated foam is a substantiallyopen celled structure and that the greater portion of its volume is voidspace. FIG. 24 shows a cross section of one of the interconnectingstrands of the coated structure. The dark triangular portion the centerof the strand is the reticulated polyurethane foam structure.Reticulated or dewindowed foam structures such as these are commerciallyavailable from Scott Paper Company and are more fully described in U.S.Pat. No. 3,l7l,820, Volz. The light portion immediately surrounding thetriangular polyurethane core is the first coating of silicone rubber.Coating the polyurethane core with silicone rubber in this mannergreatly improves the mechanical properties of the cellular structure.The first silicone coating is preferably dried and cured before applyingthe conductive coating.

As shown in FIG. 24 and in more detail in FIG. 25 the thin layer of darkmaterial surrounding the light silicone rubber portion is the conductivecoating. FIG. 25 shows that the outer surface of the coating issubstantially smooth and free from protruding particles on fiber ends.It is also apparent from FIG. 25 that the carbon particles are uniformlydistributed, over the surface of the coating. FIG. 26 is photomicrographof a portion of the surface of the coating taken normal to the surfaceand magnified to 5,000 X. The white fibers in FIGS. 26 are a fillermaterial in the silicone rubber elastic binder. The carbon particles arestill too small to be visible but they are substantially embedded in thesilicone rubber in the evenly distributed dark areas to the photo.Resistance elements such as the one shown in FIGS. 23 to 26 have beencycled as many as one million times in a fixture designed to compressthe element 42 percent of its original height and then pull the elementback to its original size with only minor decreases of 2 to 3 percent inhysteresis and linearity. It is obvious from these figures that anelastic resistance element according to this invention is far superiorthan any similar devices to date.

The preferred reticulated foam structures comprise a threedimensionalnetwork of interconnecting strands of elastic material integrallyinterconnected by nexuses at spaced points to form a skeletal outline ofa multitude of polyhedrons each face of which is polygonal and common toa pair of adjacent polyhedrons, substantially all of the faces beingopen and free from membranous elastic material. Structures having [0 tofaces per lineal inch (commonly referred to as pores per lineal inch)are preferable. The reticulated foam structure may be made of siliconerubber, thus eliminating the intermediate silicone rubber layer. Morethan one intermediate layer may be used where desirable.

' Resistance elements as illustrated by the invention characterized byan elastic material defining a collapsible structure having at least aninterconnected portion bonded with a conductive coating have many usesand advantages. The uses of the resistance element include measurementsof torsion, strain, and angular displacement as well as low-levelacoustical pickups. Transducers according to the invention find usesincluding the measurement of force, "pressure and torsion.Temperature-compensated transducers find use for measurements above roomtemperature where the combined factors of low cost and accuracy arerelevant.

One advantage of this invention is that a resistance element is providedhaving an electrical resistivity varying in a simple functional relationwith the state of tension or compression of the resistance element.

Another advantage of this invention is that a low-cost resistanceelement is provided having an electrical resistivity and sensitivitytailored to its environment and providing a reliable variation ofelectrical resistivity with continued application of tension orcompression to the resistance element.

Still another advantage of this invention is that a low-cost transduceris provided having a high output.

It will be understood that various changes in the details, materials,steps and arrangements of parts, which have been herein described andillustrated may be made within the principles and scope of theinvention.

lclaim:

1. A resistance element comprising an elastic material having aprogressively collapsible solvent-impregnated and swollen then dried andunswollen skeletal structure and a nonfriable elastic electricallyconductive continuous coating adherent to at least an interconnectedportion of said progressively collapsible structure,

said coating having an average thickness of about 0.05 to 0.25 of thethickness of said skeletal structure,

said element filling less than about 25 percent of the volume within itsouter boundary, and

said coating comprising an elastic binder and electrically conductiveparticles dispersed therein and having been applied to the structure inits solvent-swollen condition and prestressed by the subsequent dryingand shrinking to form a substantially smooth outer surface at which saidconductive particles are substantially, but not completely, embedded.

2. A resistance element as in claim 1. wherein said conductive particlescomprise about l0 to 25 percent of the volume of said coating.

3. A resistance element as in claim 1, wherein said elastic binderconsists essentially of silicone rubber.

4. A resistance element as in claim I, wherein said conductive particlesconsist essentially of carbon.

5. A resistance element as in claim 1, wherein said elastic materialconsists essentially of a nonconductive elastomer foam having aninterconnected cellular network.

6. A resistance element as in claim 1, wherein said elastic materialconsists essentially of a porous mass of particles of minim elastomerfoam adherent at their contact points, each said particle having aninterconnected cellular network.

7. A progressively collapsible variable resistance element comprising athree-dimensional network of interconnecting strands ofsolvent-impregnated and swollen then dried and unswollen elasticmaterial integrally interconnected by nexuses at spaced points to form amultitude of reticulated cells substantially all faces of which are openand free from membranous elastic material; and

a nonfriable elastic coating, substantially covering saidinterconnecting strands and nexuses without substantially filling saidopen faces, comprising an elastic binder and electrically conductiveparticles dispersed therein and having been applied to the network inits solvent-swollen condition and prestressed by the subsequent dryingand shrinking to form a substantially smooth outer surface at which saidconductive particles are substantially, but not completely, embedded insaid binder.

8. A resistance element as in claim 7, wherein the average thickness ofsaid coating is less than about 0.5 of the radius of saidinterconnecting strands.

9. A resistance element as in claim 7, wherein said network has about to100 faces per lineal inch.

10. A resistance element as in claim 7, comprising also an intermediatelayer of elastic material between the skeletal network and said coating.

11. A resistance element as in claim 10, wherein said intermediate layerconsists essentially of silicone rubber.

12. A resistance element asin claim 7, wherein the skeletal 14. Aprogressively collapsiblevariable resistance element comprising:

an interwoven structure of fibrous strands;. 5 Y

a layer of elastic material substantially covering said strands andbonding them at spaced points of intersection to form athree-dimensional interconnected solvent-impregnated and swollen thendried and unswollen network substantially free of membranous material inthe areas between strands, said elastic layer imparting elasticity tothe structure as a whole; and t a nonfriable elastic coatingsubstantially covering said network without substantially filling saidareas between strands comprising an elastic binder and electricallyconductive particles dispersed therein and having been applied to thenetwork in its solvent-swollen condition and prestressed by thesubsequent drying and shrinking to form a substantially smooth outersurface at which said conductive particles are substantially, but notcompletely, embedded in said binder.

15. A resistance element as in claim 14, wherein said layer of elasticmaterial consists essentially of silicone rubber.

16. A resistance element as in claim 14, wherein the particulateconductive material consists essentially of carbon.

2. A resistance element as in claim 1, wherein said conductive particlescomprise about 10 to 25 percent of the volume of said coating.
 3. Aresistance element as in claim 1, wherein said elastic binder consistsessentially of silicone rubber.
 4. A resistance element as in claim 1,wherein said conductive particles consist essentially of carbon.
 5. Aresistance element as in claim 1, wherein said elastic material consistsessentially of a nonconductive elastomer foam having an interconnectedcellular network.
 6. A resistance element as in claim 1, wherein saidelastic material consists essentially of a porous mass of particles ofelastomer foam adherent at their contact points, each said particlehaving an interconnected cellular network.
 7. A progressivelycollapsible variable resistance element comprising a three-dimensionalnetwork of interconnecting strands of solvent-impregnated and swollenthen dried and unswollen eLastic material integrally interconnected bynexuses at spaced points to form a multitude of reticulated cellssubstantially all faces of which are open and free from membranouselastic material; and a nonfriable elastic coating, substantiallycovering said interconnecting strands and nexuses without substantiallyfilling said open faces, comprising an elastic binder and electricallyconductive particles dispersed therein and having been applied to thenetwork in its solvent-swollen condition and prestressed by thesubsequent drying and shrinking to form a substantially smooth outersurface at which said conductive particles are substantially, but notcompletely, embedded in said binder.
 8. A resistance element as in claim7, wherein the average thickness of said coating is less than about 0.5of the radius of said interconnecting strands.
 9. A resistance elementas in claim 7, wherein said network has about 10 to 100 faces per linealinch.
 10. A resistance element as in claim 7, comprising also anintermediate layer of elastic material between the skeletal network andsaid coating.
 11. A resistance element as in claim 10, wherein saidintermediate layer consists essentially of silicone rubber.
 12. Aresistance element as in claim 7, wherein the skeletal network consistsessentially of polyurethane resin.
 13. A resistance element as in claim7, wherein the particulate conductive material consists essentially ofcarbon.
 14. A progressively collapsible variable resistance elementcomprising: an interwoven structure of fibrous strands; a layer ofelastic material substantially covering said strands and bonding them atspaced points of intersection to form a three-dimensional interconnectedsolvent-impregnated and swollen then dried and unswollen networksubstantially free of membranous material in the areas between strands,said elastic layer imparting elasticity to the structure as a whole; anda nonfriable elastic coating substantially covering said network withoutsubstantially filling said areas between strands comprising an elasticbinder and electrically conductive particles dispersed therein andhaving been applied to the network in its solvent-swollen condition andprestressed by the subsequent drying and shrinking to form asubstantially smooth outer surface at which said conductive particlesare substantially, but not completely, embedded in said binder.
 15. Aresistance element as in claim 14, wherein said layer of elasticmaterial consists essentially of silicone rubber.
 16. A resistanceelement as in claim 14, wherein the particulate conductive materialconsists essentially of carbon.